The present invention relates to equalization of wave-length-dependent optical signals, and more particularly to equalization of wavelength-dependent optical signals using a polarization-independent acoustically tuned optical filter.
It is known that long optical fiber transmission links for telecommunications can be built using cascaded chains of optical amplifiers. Erbium doped optical fiber amplifiers are particularly well-suited for implementing these long distance transmission systems due to their excellent performance characteristics and ease of fabrication.
However, multiplexed optical-signals utilizing wave-length division multiplexed (WDM) systems and erbium doped optical amplifiers exhibit a variation in signal gain that is a function of the individual wave-lengths. Moreover, utilizing cascaded optical amplifiers to compensate for attenuation over the transmission link only exaggerates the variation in signal gain for the separate wavelengths. For example, a 10 channel WDM system with a 1 nm channel spacing could easily having a gain variation over the 10 nm signal band of from 1 to 3 dB after amplification. The total gain variation is increased by the product of the number of cascaded amplifiers, and thus will certainly be much larger. While a 1 to 3 dB gain variation may be acceptable for short amplifiers chains, with 10 or more cascaded amplifiers the resulting 10 to 10 dB gain variation is not likely to be acceptable.
Large variation in component signal levels of a multiplexed signal over the wavelength spectrum complicates the design and performance of optical receivers and -detectors, and thus it is advantageous to equalize variations in signal level for any wavelength-dependent elements in the optical transmission path, particularly wavelength-dependent gain due to amplification.
Accordingly it is a primary object of this invention to obviate the above noted and other disadvantages of the prior art.
It is a further object of this invention to control the optical signal level of a optical signal composed of a plurality of differing wavelengths.
It is a yet further object of this invention to provide for uniform wavelength amplification of an optical signal composed of a plurality of differing wavelengths.
It is a still further object of this invention to provide for automatic adjustment of an optical signal composed of multiple wavelengths.
The above and other objects and advantages are achieved in one aspect of the invention by including a polarization-independent acoustically tuned optical filter (PIATOF) after a set of cascaded optical amplifiers to produce a uniform signal level for each associated wavelength of the input optical signal.
Multiple optical signals at differing wavelengths are combined by a wavelength division multiplexor and passed through a series of optical amplifiers. The output signal from the cascaded amplifiers is input to a PIATOF. A PIATOF is a two port output device, and the output of port one of he PIATOF is tapped and the tapped signal is supplied to a demultiplexer to separate the input signal according to wavelength. The resultant output signals of the, demultiplexer are input to a control circuitry. The control circuitry compares the output signal levels of the PIATOF for each wavelength and determines a proper RF power signal to be input at the control electrode of the PIATOF so that the signal level for each wavelength of the output signal at port one of the PIATOF is uniform after the amplification.